Signing Maggette or Childress in addition to getting a rental on Marion or Sheed is only a bad thing if you sign them to an untradeable contract. Otherwise they remain assets that can be used to acquire the one franchise level player this team needs either by being traded themselvs or freeing up someone like Iggy to be traded. If either one can be signed to an appropriate value contract (i.e. not a bargain or discount, not an overpayment), and possibly use the deckining salary method that the Bulls used on Nocioni and Hinrich to some extent, they would not clog our cap or hinder future moves.
Of course I don't think we should overpay for anyone, including our own RFA's. Bad contracts/bad players clog the cap, useful players with deent contracts do not. I am OK with grabbing one more core player, who is a reasonably movable piece, and one rental player this summer.
I would be most interested in how the rental deal is structured to preserve or expand our future cap space. As far as moving Miller goes, I think it is time to do it and start searching for his replacement.